Inductive power transfer or transmission is frequently used to deliver power wirelessly to portable electronic devices. Wireless power transfer is used in a variety of applications, such as, for recharging the batteries in portable devices, such as smart phones, tablets and laptops. Such power transfer systems are also used to transmit power transcutaneously, i.e., through the skin, to implanted medical devices, to either power an implant directly or to recharge the implant's battery.
As shown in FIG. 1, a conventional power transfer system 100 typically includes a coil driver 114 driving a primary coil LP (112), which inductively couples and powers secondary coil, LS (122) located inside electronic device 120. Various transcutaneous power transfer systems are described in: W. Loke, et al., “A 0.5V sub-mW wireless magnetic tracking transponder for radiation therapy,” Sym. on VLSI Cir., pp. 172-173, 2011; Y. Liao, et al., “A 3 μW wireless powered CMOS glucose sensor for an active contact lens,” ISSCC Dig. Tech. papers, pp. 38-39, 2011 and S. Lee, et al., “A low-power bidirectional telemetry device with a near-field charging feature for a cardiac microstimulator,” IEEE Tran. Bio. Cir. Syst., vol. 5, pp. 357-367, August, 2011. Although some implants are designed to obtain power directly from the batteries within the implants, those batteries that are rechargeable still have to be recharged wirelessly by an external power transmitter. See E. Lee, et al., “A biomedical implantable FES battery-powered micro-stimulator,” IEEE Tran. Cir. Syst. I, vol. 56, pp. 2583-2596, December 2009. In recent developments, many efforts have been devoted to improve the power reception and the power management within the implants. See, for example, H. Lee and M. Ghovanloo, “Fully integrated power efficient AC-to-DC converter design in inductively powered biomedical applications,” Proc. of IEEE 2011 CICC, paper 8.7, 2011. However, the coil driver in an external transmitter still requires a lot of discrete components. See the article by S. Lee, et al., 2011 cited above and also G. Kendir, et al., “An optimal design methodology for inductive power link with class-E amplifier,” IEEE Tran. Cir. Syst. I, vol. 52, pp. 857-866, May, 2005.
Class E amplifier type systems 210 as shown in FIG. 2 are commonly used in coil driver designs, See S. Lee, et al., 2011 and G. Kendir, et al., 2005 cited above. In addition to discrete capacitors CT1 (213) and CT2 (215), a bulky RF choke LC (211) is also required in this circuit topology. Since a power transmitter is normally part of a patient's external controller for an implanted medical device 220, it is important for the external controller to be small and lightweight. Therefore, a coil driver for such a wireless power transfer system should use a minimal number of discrete components to achieve a small size and should have low power consumption such that only a small battery is required.